This invention relates generally to noise-cancelling microphones. More particularly, this invention relates to a bi-directional noise-cancelling microphone that provides a broad frequency band of noise cancellation.
Microphone units typically operate in environments where unwanted acoustical components of incident sound waves are present. For example, a person listening to someone talking on the telephone may be distracted from the speaker's voice by sounds emanating from machinery, traffic, appliances, or other ambient sounds, if the person is talking into a phone without a noise-cancelling microphone.
Noise-cancelling microphones depend upon two factors for their operation. The first factor is the polar pattern of the microphone (usually bi-directional) and the assumption that the noise to be reduced is not on the maximum sensitivity axis of the microphone. The second factor is the difference in operation of a two-entry microphone for a sound source close to one entry and a sound source at a distance to the microphone.
When the sound source is close to one entry, the sound pressure will be several times greater at the close entry than at the remote entry. Since the microphone responds to the difference of sound pressure at the two entries, close talking will provide a substantially higher sensitivity than a remote sound, where the sound pressure is equal in magnitude at the two entries.
Small microphones with tubular sound entries have been used for some time, particularly in hearing aids. This has the advantage that the sound entries may be placed in an appropriate location on the hearing aid case without regard to accommodating the bulk of the microphone cartridge. Additionally, the tubes need not be straight, but may have unsymmetrical bends. Typically, the tubes are of approximately equal length.
There are two acoustic parameters which affect the output. The first is the distance between entries. The maximum acoustic input will occur at the frequency where the distance between the entries is a half wavelength. Ordinarily, this distance will be between 1/2" and 3", with the shorter distances giving higher peak frequencies and lower general noise sensitivity.
The other parameter is the total length of the microphone tube. When this distance is a half wavelength, the two halves of the tube will exhibit 1/4 wave "organ pipe" resonances which determine the highest practical frequencies passed to the microphone element.
Since the total length of the tube is always the same or larger than the distance between the entries, the tube length determines the maximum practical frequency response, both for noise and for a close talking sound source at one entry of the microphone.
In the practical use of this microphone, especially in a retrofit to an existing case, the total tube length can become 3 or 4 inches, which would restrict the frequency response to 2 KHz or lower. This is an unfortunate restriction, since much information is contained in high frequency sibilant sounds.
Two attempts in the prior art to produce noise-cancelling microphones are illustrated in U.S. Pat. No. 3,995,124, issued to Gabr, and U.S. Pat. No. 4,950,016, issued to Groves, et al. In the Gabr patent, the sound responsive element of the microphone, a diaphragm, is designed to have both of its sides exposed substantially equally to unwanted noise. In the Groves, et. al. patent, separated sound paths are provided from the exterior of a telephone handset to the front and rear surfaces of a diaphragm, thereby reducing the effect of unwanted noise signals. Neither of these patents suggests the unique noise-cancelling microphone of this invention.